1.Field of the Invention
The present invention relates to a pulse doppler radar system with variable pulse repetition rate, and especially to such a system having an arrangement for cancelling permanent echos and for integrating the reflected pulses.
2. The Prior Art
Various systems have been developed for ranging, moving and fixed targets using doppler radar, and such systems which have fixed pulse repetition rates are adapted to processing unwanted multiple-time-around echos.
When a pulse doppler radar system is employed with periodic switching of the pulse repetition rate, using a doppler filter to furnish a moving target indication, the cancellation of permanent echos from multiple-time-around ranges deteriorates. Whereas fixed echos from the unambiguous range are returned unaltered at various pulse repetition rates, the permanent echos from multiple-time-around ranges fall into different ranges, so that the permanent echo amplitude usually changes in every range. Accordingly, the basis for cancelling permanent echos in the doppler filter is lost, and the varying amplitude permanent signals can no longer be discriminated from moving target echos signals, and can only partially be cancelled by the doppler filter.
Due to the change of the pulse repetition rate from pulse to pulse (sometimes referred to as staggering), the echos of individual pulses, or noise signals that simulate reflected target pulses from multiple-time-around ranges fall into different range gates, or individual ranges. Therefore, the single pulses falling into the several range gates cannot be integrated. The probability of detection of such pulses is considerably reduced in the presence of superimposed noise. When an extensive clutter region extends over several ranges, the clutter corresponds to signals representing a plurality of distance ranges for the radar system, and then the probability that a plurality of target echos from a plurality of multiple-time-around ranges will accumulate within the same range gate rises considerably. In this case, permanent echos that derive from multiple-time-around targets can lead to erroneous operation.
As described in a German patent DE OS 3,110,279, it is known to stagger the pulse repetition rate such that at least two different pulse repetition rates are transmitted during the target sweep time. Targets from multiple-time-around ranges are distinguished from those in the unambiguous range because the echo signals from each individual pulse repetition fall into a different range gate. As a result, the permanent echos supply fewer signals to be integrated in accord with the number of pulse repetition rates employed and are attenuated in comparison to signals from the unambiguous range of the radar system. Since every switchover of the pulse repetition rate leads to transients in the moving target filter, which transients result in changes in the amplitude of permanent echos and thus disrupt the moving target indication, the first reception periods after each changeover of the pulse repetition radar are blanked out. Without a noticable loss of sensitivity, this type of staggering of the pulse repetition rate is particularly suitable for radar installations that have a high number of reflected signals during the target sweeptime. Because of cost consideration, the integration of the echo signals does not usually take place in a coherent fashion. Circuits which integrate the echo signals after they are received are relatively inexpensive but do not achieve an optimum effect.
Accordingly, it is desirable to provide a system with pulse doppler radar of the type having periodic switching of the pulse repetition rate in which deterioration of the moving target indication is prevented.